Consistent Terminology for Solvation

There seems to be a need of a clear defined vocabulary to name and differentiate between the processes of hydration, solvation and dissolvation. Particular when handling solvation free energy. Even in literature (and especially in papers) seems to be no consistent use of such terms.

That is why this collection of terms should be obligatory for our group, to be consistent when working with each other and writing paper.

This page is merely a compendium for terms to use. If you like to get some deeper insight about the physics, you may want to read some good book or visit this page for a short overview about solution.


Definitions of Components and Processes


A liquid or solid phase containing more than one substance, when for convenience one (or more) substance, which is called the solvent, is treated differently from the other substances, which are called solutes. When, as is often but not necessarily the case, the sum of the mole fractions of solutes is small compared with unity, the solution is called a dilute solution. iupac

remark: it is worth to stress the fact, that the distinction between solvent and solute in merely arbitrarily.


The minor components of a solution which are regarded as having been dissolved by the solvent. iupac


The major components of a solution which are regarded as having dissolved a solute.

remark: the terms solute and solvent can refer to one or many different molecular species.

dilute solution

Solution in which the sum of mol fractions of all the solutes is small compared to 1.

remark: If you dilute a solution by adding solvent, you do not get a diluted solution (although it would be correct english) but a dilute solution.

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Any stabilizing interaction of a solute (or solute moiety) and the solvent or a similar interaction of solvent with groups of an insoluble material (i.e. the ionic groups of an ion-exchange resin). Such interactions generally involve electrostatic forces and van der Waals forces, as well as chemically more specific effects such as hydrogen bond formation. iupac

hydration (physical)

The physical meaning of the term hydration is important for us.

The term is also used for the process:
A (gas) --> A (aqueous solution)
cf. the use of the term in inorganic/physical chemistry to describe the state of the ions of an electrolyte in aqueous solution. iupac

remark: hydration can be seen as a special case of solvation, where the solvent is water.
2nd remark: also consider the difference in solvation shell and hydration shell.

As you may guessed, hydration refers not to hydrogen atoms, but explicitly to water.

hydration (chemical)

The chemical meaning of hydration is introduced for the sake of completeness and to avoid misunderstandings.
Addition of water or of the elements of water (i.e. H and OH) to a molecular entity. For example, hydration of ethene:
CH2 = CH2 + H2O --> CH3-CH2-OH iupac


The mixing of two phases with the formation of one new homogeneous phase (i.e. the solution). iupac

remark: For the dissolution of an crystal in a solvent, dissolution include the whole process of removing the ions from the crystal lattice and solvate them in the solvent.


The direct transition of a solid to a vapour without passing through a liquid phase. Example: The transition of solid CO2 to CO2 vapour. iupac


Clusterication of a solute molecule with a few solvent molecules.

remark: This is no rigorous defenition from IUPAC but more a picture to have in mind, when reading this term.

A remark on Verbs

There is also a pitfall using the associated verbs.

A solute is dissolved within a solvent, forming the solution.
A mathematical equation is solved, giving also a solution.
A solute can be solvated in a solvent, and hydrated in water.

Definition of (Gibbs) Free Energy

As mentioned above, when handling free energy differences, a precise definition is necessary. In an ideal case, for every calculated free energy difference should be a definition of initial and final state to avoid any misunderstandings (with you and the reader or with you and the auditory).

Free energy for itself is misleading, because there are two energies termed with free energy: the Gibbs free energy G see wiki (or Gibbs function iupac) and the Helmholtz free energy A (or F) see wiki (or Helmholtz function iupac). In the context of solvation, we will use free energy for the Gibbs free energy, but it may be a good idea to always use the full term, give a clear definition beforehand or add the corresponding symbol (like G).

dissolution process, devided in sublimation free energy and solvation free energy

solvation (free) energy (ΔGsolv)

The change in Gibbs energy when an ion or molecule is transferred from a vacuum (or the gas phase) to a solvent. The main contributions to the solvation energy come from:
1. the cavitation energy of formation of the hole which preserves the dissolved species in the solvent;
2. the orientation energy of partial orientation of the dipoles;
3. the isotropic interaction energy of electrostatic and dispersion origin; and
4. the anisotropic energy of specific interactions, e.g. hydrogen bonds, donor-acceptor interactions etc. iupac

remark: In contrast to IUPAC, we will term this energy solvation free energy.

Below: a scheme, illustrating the procedure of solvation which may be used in computer simulation, taken from J. Phys. Chem. B 2007, 111, 1872-1882.
Creating a cavity in the solvent
insertion of the solute
turning on the partial charges

Hydration free energy (ΔGhyd)

is the change of the Gibbs free energy that accompanies the transfer of a solute from gaseous phase to water.

Free energy of solution (or dissolution) (ΔGsoln)

is the change of the Gibbs free energy that accompanies the transfer of a solute from crystal to a solvent.

remark: the term dissolution is more associated with a kinetic property, whereas we are mostly interessted in the properties of equilibrated states. That's why free energy of solution is preferable to use.

Sublimation Free energy (ΔGsub)

is the change of the Gibbs free energy that accompanies the transfer of a solute from crystal to gaseous phase.

Naturally ΔGsoln = ΔGsub + ΔGsolv.

standard condition and standard state

The Gibbs free energy is a function of a system at a specific state. It is natural to define a standard state to correspond. This was done by IUPAC, but in a quite complex way:

State of a system chosen as standard for reference by convention. Three standard states are recognized: For a gas phase it is the (hypothetical) state of the pure substance in the gaseous phase at the standard pressure , assuming ideal behaviour. For a pure phase, or a mixture, or a solvent in the liquid or solid state it is the state of the pure substance in the liquid or solid phase at the standard pressure. For a solute in solution it is the (hypothetical) state of solute at the standard molality, standard pressure or standard concentration and exhibiting infinitely dilute solution behaviour. For a pure substance the concept of standard state applies to the substance in a well defined state of aggregation at a well defined but arbitrarily chosen standard pressure. iupac

In most cases, we study the system under standard conditions (STP). But even there, some deviations exist. There is a nice table in wikipedia table of STP. The most convenient way would be, to always state your specific conditions of your simulation/ experimental data instead of mention 'under standard condition'.

If the notation ΔG0 is used, the change in the Gibbs free energy between one state and a state under standard condition is meant. A state under standard condition in this context is a pure compound A at a temperature T and a pressure P. For example when dissolving a pure crystal NaCl under STP in water, the change in Gibbs free energy would be notated with ΔG0.

remark: the definition and usage of ΔG0 has to be extended.

Common Units of Gibbs free energy

As a thermodynamical potential, the Gibbs free energy is given in units of energy, say kJ, kcal or eV. More commonly used for changes in the Gibbs free energy of systems is the energy value per mol, where 1 mol = 6.022×10^23 molecules. When considering solvation free energy of single molecules the unit eV per molecule is also used. The unit calorie (cal) is no SI unit and has different definitions see wiki.

1 eV/molecule = 96.485 kJ/mol
1kcal = 2.611×10^19 eV = 4.184 J